PROJECT SUMMARY Converging evidence indicates that neuronal and network hyperexcitability is an important early event in Alzheimer disease (AD) patients. The cellular and molecular basis of this hyperexcitability is a critical area of investigation and the presence of similar hyperexcitability in animal models enables studies to dissect underlying mechanisms. A key insight is that hyperexcitability in both AD patients and mouse models has a strong diurnal rhythm. Emerging data from both humans and animal models indicate that neural excitability in the forebrain is under circadian control, altering seizure thresholds and epileptiform activity. Circadian variation in cellular function is driven by transcriptional molecular clocks expressed in most cells, and molecular clock ablation increases AD pathology. We have compelling preliminary evidence for rhythmic variation in neuronal excitability that is at least partly due to circadian regulation of the membrane properties of inhibitory interneurons, especially fast-spiking cells that express parvalbumin ? a cell type implicated in AD. Given that molecular and physiological rhythms in hippocampus are disrupted in AD patients and AD mouse models, we propose rigorous experiments to test the hypothesis that dysregulation of the molecular clock and resulting changes in PV+ interneuron gene expression and activity contributes to AD-related neuronal hyperexcitability. Specifically, we will evaluate the differences in circadian clock and clock-controlled gene expression in PV+ interneurons in a mouse model of AD, using a combination of RNA sequencing, state-of-the-art bioinformatics, and recently developed tools to evaluate molecular clock rhythmicity and transcription in a cell-specific manner (Aim 1). We will use patch-clamp electrophysiology to determine if AD-related impairment of the circadian clock alters day-night differences in neurophysiological properties of PV+ interneurons, causing hyperexcitability (Aim 2). Finally, we will utilize an innovative chemogenetic chronotherapeutic approach to manipulate PV+ interneuron physiology to determine whether reinstating the normal circadian regulation of PV+ interneuron electrophysiological properties protects against AD-related hyperexcitability, cognitive impairment, and pathology (Aim 3). The proposed studies led by a strong interdisciplinary team uses powerful approaches to determine how disruption of circadian rhythms facilitates neuronal hyperexcitability that contributes to early stages of AD. Understanding these mechanisms may catalyze development of behavioral or pharmacologic interventions.